The present invention relates to a toner for developing an electrostatic image in an image forming method such as electrophotography, electrostatic recording or electrostatic printing and, more particularly, to a dry toner for use in an image forming apparatus, such as a copying machine, a laser printer or a facsimile machine. Moreover, the present invention is also directed to a method of preparing the above toner.
A developer for use in electrophotography, electrostatic recording, electrostatic printing and so on is once adhered to an image carrier such as a photoconductor on which an electrostatic latent image has been formed in a developing process. The toner image thus obtained is then transferred from the photoconductor to a transfer medium such as a transfer paper in a transfer process, and fixed on the paper in a fixing process. As a developer for developing the electrostatic image formed on a latent image holding surface of the image carrier, a two-component developer composed of a carrier and a toner and a one-component developer requiring no carrier (magnetic or nonmagnetic toner) are known.
As a dry toner for use in electrophotography, electrostatic recording, electrostatic printing and so on, a toner obtained by melt-kneading a toner binder such as a styrene resin or a polyester together with a colorant and so on and finely pulverizing the kneaded mixture is conventionally used.
After having been developed and transferred to a paper or the like, such a dry toner is heat-melted and fixed with a heat roll. At this time, when the temperature of the heat roll is excessively high, the toner is excessively melted and adhered to the heat roll (hot offset). When the temperature of the heat roll is excessively low, on the other hand, the toner is not sufficiently melted, resulting in insufficient fixation. With a view to energy saving and downsizing of an apparatus such as a copying machine, a toner which does not cause hot offset at a high fixing temperature (namely, has hot offset resistance) and which can be fixed at a low fixing temperature (namely, has low temperature fixability) is demanded. The toner should also have heat-resistant preservability so as not to cause blocking during storage or under ambient temperature in an apparatus in which the toner is used. Especially, a toner for use in a full-color copying machine and a full-color printer need to have a low melt viscosity to provide gloss and color mixability in a printed image, so that a polyester type toner binder having a sharp melt property is used therein. Since such a toner is likely to cause hot offset, a silicone oil or the like is conventionally applied to a heat roll in full-color machines. However, in order to apply a silicone oil to a heat roll, an oil tank and an oil applying unit are necessary, which makes the apparatus unavoidably complicated and large. Also, application of oil causes deterioration of the heat roll, so that the heat roll requires regular maintenance. Additionally, it is unavoidable for the oil to adhere a copying paper and an OHP (overhead projector) film. Especially, the oil adhered to OHP film impairs color tone of a printed image.
For the purpose of producing an image with high fineness and high quality, improved toners having a small particle size or a narrow particle size distribution have been proposed. However, particles of a toner produced by a normal kneading-pulverizing method have irregular shapes. Thus, the toner particles are further pulverized to generate superfine particles or a fluidizing agent is buried in the surface of the toner particles when the toner is agitated with a carrier in a developing unit or when, in the case of being used as a one-component developer, the toner particles receive a contact stress from a developing roller, a toner supply roller, a layer thickness regulating blade, a frictional electrification blade and so on, resulting in deterioration of image quality. Also, the toner is poor in fluidity as a powder because of the irregular shapes of the particles thereof, and thus requires a large amount of fluidizing agent or cannot be filled in a toner bottle with a high filling rate, which hinders downsizing of the apparatus.
Additionally, a process of transferring an image formed of color toners to produce a full-color image from a photoconductor to a transfer medium or a paper is becoming more complicated, so that low transferability of a pulverized toner due to the irregular shapes of the particles thereof causes a void in a transfer image and an increase in consumption of toners to prevent it.
Thus, there is an increasing demand for reducing toner consumption without causing a void in a transferred image by improving transfer efficiency and for decreasing running cost. When transfer efficiency is significantly high, there is no need for a cleaning unit for removing untransferred toner from a photoconductor and a transfer medium, which leads to downsizing of the apparatus and cost reduction in manufacturing the same. This has also a merit of generating no waste toner. For the purpose of overcoming the drawbacks of the toner of irregular particle shape, there has been proposed various methods for producing spherical toner particles.
For the purpose of providing a toner having heat-resistant preservability, low-temperature fixability and hot offset resistance, there have been proposed (1) a toner in which a polyester partially crosslinked using a multifunctional monomer is used as a toner binder (Japanese Laid-Open Patent Publication No. S57-109825) and (2) a toner in which a urethane-modified polyester is used as a toner binder (Japanese Examined Patent Publication No. H07-101318). For the purpose of providing a toner for use in a full-color system which can reduce the amount of oil to be applied to the heat roll, (3) a toner produced by granulating polyester fine particles and wax fine particles is proposed (Japanese Laid-Open Patent Publication No. H07-56390). Proposed for the purpose of providing a toner having improved powder fluidity and transferability when its particle size is reduced are (4) a polymerized toner obtained by dispersing a vinyl monomer composition containing a colorant, a polar resin and a releasing agent in water and suspension-polymerizing the vinyl monomer composition (Japanese Laid-Open Patent Publication No. H09-43909) and (5) a toner obtained by sphering toner particles comprising a polyester type resin in water using a solvent (Japanese Laid-Open Patent Publication No. H09-34167). Additionally, Japanese Laid-Open Patent Publication No. H11-133666 discloses (6) a dry toner consisting of nearly spherical particles in which a polyester modified with a urea bond is used as a bonder. The toner has a Wadell practical sphericity of 0.90 to 1.00.
However, none of the toners (1) to (3) have sufficient powder fluidity and transferability and thus can produce a high-quality image even when its particle size is reduced. The toners (1) and (2) cannot compatibly satisfy the heat-resistant preservability and the low temperature fixability and do not develop sufficient gloss to be used in a full color system. The toner (3) is insufficient in the low-temperature fixability and the hot offset resistance in oilless fixations. The toners (4) and (5) are improved in the powder fluidity and the transferability. However, the toner (4) is insufficient in the low-temperature fixability and requires much energy to fix. This problem is pronounced when the toner is used in full-color printing. The toner (5), which is superior to the toner (4) in the low-temperature fixability, is insufficient in hot offset resistance and thus cannot preclude the necessity of the application of oil to the heat roll in a full-color system.
The toner (6) is excellent in that the viscoelasticity of the toner can be appropriately adjusted by using a polyester extended by a urea bond and that it can compatibly satisfy both good gloss and good releasing property as a full-color toner. Especially, a phenomenon called “electrostatic offset” in which unfixed toner on a transfer medium is scattered or adhered to a fixing roller due to electrification of the fixing roller during use can be reduced by neutralization of positive charges of the urea bond component with weak negative charges of the polyester resin. However, it has been found that the toner having a Wadell practical sphericity of 0.90 to 1.00 practically contains still causes degradation of image quality.